Awatere dyke swarm, Marlborough, New Zealand

Abstract

The Awatere dyke swarm intruded near surface (3-7 km deep) Mesozoic sedimentary rocks of the Inland Kaikoura Ranges between 100-95 Ma. The myriad of near-vertical, basalt-trachybasalt-tephrite basanite dykes were coeval with the Tapuaenuku plutonic complex and erupted as the Lookout Formation. At a distance of >4 km from the Tapuaenuku plutonic complex, the dykes align with the inferred regional greatest compressive palæostress direction, indicating N-NNE mid Cretaceous extension. Total crustal dilation by dyke material averages 11% but ranges from 7-24% at a palaeodepth of 4.5 km to 4-11% near the Lookout Formation (palæosurface). Intrusion of the Awatere dyke swarm instigated E to SE striking, reverse and normal small-scale (cm-m offset) faulting within the host rock Gladstone and Winterton Formations. Contraction and extension was experienced in the pre-Cenozoic N to NE direction. Movement on the Winterton Fault, a large low-angle normal fault with 3.0±0.5 km dip-slip separation in the NNE-SSW direction, is interpreted to have occurred immediately prior to, and possibly coeval with the intrusion of the Awatere dyke swarm. Transport directions for mesoscopic fault planes thought to represent the Winterton Fault indicate a NE-SW extension direction which pre-dated the intrusion of at least some of the Awatere dyke swarm. Small-scale (cm-m offset) faulting associated with movement on the Gladstone fault, a Cenozoic, predominantly dextral, strike-slip fault with an offset of 1.5±0.5 km, show a SE-NW contraction, comparable to the geodetically determined strain axis of 100-128° for the Awatere to Clarence area (Bibby, 1976). No evidence was found of regional deformation of the Winterton River area since the intrusion of the Awatere dyke swarm, except for a regional tilting to the NW of ~26° which exposed this section of crust as the Inland Kaikoura Range. The Awatere dyke swarm has not undergone any post-intrusional folding by the compaction of sedimentary host rocks as was previously thought. The apparent folds or undulations occurred during dyke propagation, before the magma quenched.